Today's mobile devices, smart phones, electronic books, tablet computers, and laptop computers (generically referred to herein as “host devices”) typically include a host device panel that comprises a large area of glass (i.e., a glass panel, at least part of which may function as a display and/or input device) constituting a surface of the device. The host device panel may comprise a display device and/or an input device (e.g., a touch screen). Many manufacturers of such devices are keen to incorporate fingerprint sensors into those devices to facilitate user authentication and access to the device. Capacitive fingerprint sensors offer a cost-effective solution. Thermal, ultrasonic or optical fingerprint sensors are alternative solutions.
Fingerprint sensors may not perform well if placed under glass panels of the type typically used in host devices. Glass that provides the necessary mechanical robustness required for a portable, practical consumer device may be too thick and/or may have electrical properties that are not necessarily optimal for the fingerprint sensor to meet specified acceptance/rejection rates. Putting sensors behind the display glass causes a significant and typically unacceptable degradation of the signal to noise ratio (“SNR”) for the sensor, and likewise causes increased blurring of the imaged (sensed) fingerprint.
Some manufacturers have used a dedicated cover member that covers the fingerprint sensor only, such as a sapphire window. The host device panel of the host device fits around the dedicated sapphire window. Sapphire is very expensive, however, and therefore increases material costs of the host device substantially.
Another solution in development by several manufacturers is to take a glass host device panel and then machine away or etch away an area that will cover the fingerprint sensor to locally reduce the thickness (i.e. to “thin”) that portion of the panel. However, this adversely affects the strength of the glass panel in the thinned region, and can create substantial stress concentration at the transition region between the full thickness glass pane and the thinned region. The thinning process is also costly and difficult to control. It is also difficult to get this thinned region of glass thin enough to prevent excessive degradation of the fingerprint image, both in terms of signal to noise ratio (“SNR”) and blurring, and any defects in the transition area from thin to thick glass created in the thinning process can dramatically increase the probability of failure in this region, thereby jeopardizing the robustness of the glass panel.
The present applicant has developed a capacitive fingerprint sensor that will operate effectively, including in terms of SNR and blurring, behind a cover member made of glass, such as strengthened, boroaluminosilicate glass, rather than sapphire, and furthermore is able to meet mechanical robustness requirements such as drop tests, shock and vibration tests, ball drop tests, tumble tests, etc.
This disclosure describes practical manufacturing methods of manufacturing the cover member.
U.S. Provisional Application Nos. 62/258,284; 62/349,256; and 62/374,339, “Electronic Sensor Supported on Rigid Substrate,” and U.S. Patent Application Publication No. 2017/0147852 claiming priority thereto, the respective disclosures of which are hereby incorporated by reference, describe “wrapped sensor” designs covered with a glass cover member. Such wrapped sensor designs comprise a rigid substrate wrapped in a flexible circuit subassembly comprising conductive trace sensor elements, a circuit element such as an integrated circuit or application specific integrated circuit (“ASIC”), and conductive interconnects connecting the sensor elements to the circuit element all disposed on a flexible substrate material.
U.S. Provisional Application No. 62/354,210, “Reinforcement Panel for Fingerprint Sensor Cover” and U.S. patent application Ser. No. 15/628,003 (U.S. Patent Application Publication No. 2017/0372112) claiming priority thereto, the respective disclosures of which are hereby incorporated by reference, describe “wrapped sensor” designs covered with a glass cover members reinforced with a ceramic layer to aid robustness.